Screen printable resistor compositions compatible with nitrogen (or low oxygen partial pressure) fireable conductors are relatively new in the art of thick film technology.
Thick film resistor composites generally comprise a mixture of electrically conductive material finely dispersed in an insulative glassy phase matrix. Resistor composites are then terminated to a conductive film to permit the resultant resistor to be connected to an appropriate electrical circuit.
The conductive materials are usually sintered particles of noble metals. They have excellent electrical characteristics: however, they are expensive. Therefore, it would be desirable to develop circuits containing inexpensive conductive materials and compatible resistors having a range of stable resistance values.
In general, nonnoble metal conductive phases such as Cu, Ni, Al, etc. are prone to oxidation. During the thick film processing, they continue to oxidize and increase the resistance values. However, they are relatively stable if the processing can be carried out at low oxygen partial pressure or "inert" atmosphere. As used herein, low oxygen partial pressure is defined as the oxygen partial pressure that is lower than the equilibrium oxygen partial pressure of the system consisting of the metal conductive phase and its oxide at the firing temperature. Therefore, development of compatible resistor functional phases which are capable of withstanding firing in a low oxygen partial pressure without degradation of properties is the prime objective in this technology. The phases must be thermodynamically stable after the processing of the resistor film and noninteractive to the nonprecious metal terminations when they are cofired in an "inert" or low oxygen partial pressure atmosphere. The major stability factor is the temperature coefficient of resistance (TCR). The materials are considered stable when their resistance values do not change appreciably when the resistor components are subjected to temperature changes.